Thesis (Ph.D.) - University of Queensland, 2003. / Includes bibliography.

The effect of aluminium additions on the structure and morphology of the corrosion products formed on the surfaces of rapidly solidified Mg-Al alloy splats immersed at room temperature in a solution of 3%NaCl saturated with Mg(OH)2; has been studied under different analytical techniques. The adverse effect of contamination from copper particles during processing on the corrosion behaviour of the alloys is also highlighted. Aluminium additions were beneficial to the corrosive behaviour of the alloys with a marked improvement in their anti-corrosion resistance occurring in alloys containing more than 10 wt.% Al. This is attributed to the presence of aluminium ions in the prior oxide/hydroxide in the surface of the alloy. The thickness of the latter decreased with enrichment of aluminium ions and was 10-50nm for the Mg-16Al alloy splats as compared with 200nm for the Mg-3.5Al alloy splats. Hydromagnesite (3MgCO3. Mg(OH)2. 3H2O) formed as an overlayer on the surface of the alloy splats depending on the handling conditions. For the Mg-10Al and Mg-16Al alloy splats an admixture of a high temperature spinel (MgA12O4) in perlclase (MgO) and/or brucite (Mg(OH)2) was detected by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). It Is proposed that in the corrosive environment the Al3+ ions on the surface compete successfully with the chlorine ions for the anodic sites on the surface and anchor the growth of the layered brucite structure by the formation of a compound belonging to the pyroaurite-sjogrenite group of compounds. Hydroxyl ions, water, chlorine ions and carbonate ions are incorporated in The interlayers of the layered brucite structure. The formation of a double hydroxide with an acicular morphology and a structure close to that of hydrotalcite-manasseite (Mg6A12(OH)16. CO3. 4H2O) has been supported by scanning transmission electron microscopy (STEM), XPS, XRD, multi-element mapping by electron probe microanalysis (EPMA) and Rutherford backscattering spectrometry (RBS) analyses on the corroded splats. A growth mechanism is proposed on the basis of the structural chemistry, surface morphology and crystal structure of the corrosion products. The implications of this work for the design of Mg base alloys with improved corrosion properties are also discussed. The selected alloying elements are in excellent agreement with those selected from other studies on the development of corrosion resistant Mg alloys.

"Research and development of magnesium casting alloys depends largely on the metallurgist’s understanding and ability to control the microstructure of the as-cast part. Currently few sources of magnesium solidification information and as-cast microstructures exist. Therefore, the goal of this research is to increase the general knowledge base of magnesium solidification behavior and to characterize the resultant microstructures. Equipment has been developed and constructed to study the solidification behavior of magnesium-aluminum casting alloys via non-equilibrium thermal analysis and continuous torque dendrite coherency measurements. These analyses have been performed on six magnesium-aluminum alloys, including industry dominant alloys such as AM60 and AZ91E, and experimental alloys which show commercial potential such as AXJ530. The resultant microstructures have been characterized for general microstructure trends and the various phases present were analyzed using optical and scanning electron microscopy, as well as energy dispersive x-ray spectroscopy. The measurements were performed using a cooling rate on the order of 1-2°C/s, and results of these analyses show that in general, magnesium-aluminum casting alloys have relatively large solidification ranges, non-dendritic microstructures, and coherency points that are similar to those of aluminum casting alloys. These results should prove useful for research directed towards development of new magnesium alloys that are targeted for specific applications, as well as for optimizing casting procedures for Mg-Al alloys to obtain defect free cast structures."

Aluminum 5XXX alloys are of industrial importance and interest as they combine a wide range of desirable strength, forming and welding characteristics with a high resistance to corrosion. The presence of Mg in these alloys ensures favorable mechanical properties. However, the room temperature stretching performance of these alloys is limited. Moreover, Al-Mg alloys are known for being susceptible to the Portevin-LeChatelier effect when deformed at room temperature. Nevertheless, improvements in ductility can be achieved through warm forming, especially when the ductility approaches superplastic levels. / The aim of this study was to test for enhanced ductility in three coarse-grained Al-Mg alloys namely, super-pure Al-3%Mg and Al-5%Mg, and commercial AA 5056 alloy. The temperature-dependent flow stress and rate sensitivity behavior of these alloys was investigated by means of tensile testing using ASTM E8M-04 standard samples. Samples were deformed to 10% strain to allow enough deformation to occur such that serrations in the dynamic strain aging (DSA) temperature/strain rate range would be rendered visible on a stress-strain curve. Using this information, the regions of negative and higher-than-normal strain rate sensitivity ('m') were plotted and tensile tests to failure were performed in the vicinity of maximum 'm'. ASTM E2448-06 standard samples for superplasticity tensile testing were used in this case. / A maximum ductility of 170% was recorded with these samples and this was found to increase to nearly 300% when the gage length was shortened. It was observed that the DSA serrations were more prominent at lower strain rates, higher temperatures and higher Mg contents. The results of this study show clearly that if the rate sensitivity is high enough, then enhanced ductility in coarse-grained materials is possible at temperatures well below the maximum test temperature.

Thesis (M.S.)--University of Missouri--Rolla, 2007. / Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed June 17, 2008) Includes bibliographical references.

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Thesis (M. Phil.)--University of Hong Kong, 2008. / Includes bibliographical references. Also available in print.

Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: microstructures; magnesium alloys; solidification behavior. Includes bibliographical references (p. 106-108).